Structural component coated with a hard material and comprising an intermediate layer for improving the adhesive strength of the coating

ABSTRACT

Structural components that are subject to high stresses, especially caused by wear, can be protected by coating them with a hard material. For metal-cutting tools it is for example known that their wear resistance can be improved by coating them with a hard material. Different thermal expansion coefficients of the substrate material and the coating material and different layer materials in a multilayer coating may produce great stresses. The stress increases as the thickness of the coating increases, thereby increasing the danger of the coating chipping off. In order to solve this problem, the invention proposes an intermediate layer ( 5 ) to be interposed between at least two layers ( 4, 9 ) of the coating ( 3 ), said intermediate layer mainly or exclusively consisting of a material component ( 6, 8 ) of an element of the fourth to eighth subgroup of the fourth and fifth period of the periodic system.

[0001] The invention relates to a structural component made of a metallic or ceramic material and having a coating of hard material according to the pre-characterising part of the first claim, as well as a process for the application of this coating according to the thirteenth claim.

[0002] Structural components that are subjected to high loads, in particular caused by wear, can be protected by a coating of hard material. In particular the tribochemical and thermochemical reactions between the materials of the work piece and the tool must be largely avoided so that the good mechanical and thermal properties of the material may be exploited to the full. It is therefore already known to improve the wear resistance of, for example, metal-cutting tools by means of a coating of hard material. The coatings are applied not only to metal-removing tools made of metal, in particular high-speed steel (HSS) or hard metal (HM), but also to tools made of ceramic materials, such as, for example, silicon nitride blanking dies. Wear can also be reduced by a coating of hard material in the case of metal-forming tools.

[0003] Layer thicknesses within the micrometer range generally suffice for the desired property improvements, in particular in the case of metal-cutting tools. Such layers can be deposited, for example, from the gas phase by the PVD, CVD or plasma CVD processes. Depending on the material, layer thicknesses of a layer of less than 1 μm to over 20 μm are possible by these means.

[0004] The processes indicated enable not only single layers of one material to be deposited. Many variations are possible as regards both the number of materials which may be combined together in the layer sequence, and the number of layers. Above all when metal-cutting tools are utilised, layer materials based on titanium, such as, for example, titanium nitride, titanium carbide, titanium oxide, titanium carbon nitride are preferably utilised, as well as layers of one or more modifications of aluminium oxide.

[0005] Differences in thermal expansion between the substrate material and the layer material, as well as different layer materials in the case of a coating comprising a plurality of layers, may give rise to high stresses. In particular with increasing coating thickness the stress increases, as consequently does the risk of the coating chipping off.

[0006] In order to improve the adhesion strength of a coating it is known, for example from DE 36 08 734 C1, in the case of coated ceramic indexable inserts to deposit an intermediate layer of silicon dioxide on a base body made of sintered ceramic materials which is provided with a ceramic wear layer. This prior art, it must be noted, relates only to coating with one layer.

[0007] The object of the present invention is to increase the adhesion strength of the layers of hard material on the substrate material of a structural component made of a metallic or ceramic material as well as that between the layers.

[0008] The object is achieved in a structural component according to the pre-characterising part of the first claim with the aid of the characterising features thereof. In the thirteenth claim the process according to the invention for the production of these structural components is claimed. Advantageous embodiments are claimed in the dependent claims.

[0009] A structural component according to the invention having a coating is distinguished in that at least between two layers of the coating an intermediate layer is arranged which comprises predominantly or exclusively a material component of an element in the fourth to eighth sub-group of the fourth and fifth period of the Periodic Table. The number of intermediate layers is matched to the number of layers of the coatings. The thicker a coating is and the greater the number of layers necessary for building up the layer, the more advantageous is the provision of intermediate layers.

[0010] In particular the elements from the eighth sub-group of the fourth period are suitable for the formation of an intermediate layer. For example, in the case of layers based on titanium the adhesion strength is preferably increased by intermediate layers comprising a material component of the element cobalt.

[0011] A coating of hard material is applied in known manner by the PVD, CVD or plasma-CVD processes from the gas phase. The intermediate layer according to the invention is likewise deposited from the gas phase. For this purpose, the element of the intermediate layer is likewise converted to the gas phase. If, for example, a layer based on titanium such as Ti(C,N) is built up by the CVD process from the gas phase, a pressed body may be introduced into the reactor in order to form an intermediate layer based on a metallic material. The deposition of a layer, for example based on titanium, preferably takes place at a temperature of approximately 1000° C. and at a pressure slightly below atmospheric pressure. Under these conditions the metal remains in the gas phase and is not deposited as a layer material. Only when the pressure is reduced is the intermediate layer built up with the metallic material component. This intermediate layer may, depending on the parameters adjusted during coating, or the available quantity of the metallic material component, be embodied as a cohesive layer or be present in the form of interstitial regions. If the metallic material component supply is exhausted, or if the pressure is again raised to the previous level, the intermediate layer also stops building up. These intermediate layers are thinner than the other layers and have a thickness of from approximately 0.05 μm to approximately 5 μm. The stresses within the coating are reduced by an intermediate layer, thus substantially lowering the risk of chipping-off.

[0012] The layers generally have a composition such as those which arise in accordance with the known coating processes. Coatings based on titanium generally comprise the following compounds: TiN, Ti(C,N), TiC, TiO, TiON and TiCON.

[0013] The coatings may, however, also comprise at least one of the modifications of aluminium oxide.

[0014] The coating may be single-phase, that is to say may comprise one material. The coating may also be multi-phase, that is to say may consist of an alternating sequence of layers of different materials, for example based on titanium, and aluminium oxide. Such a coating comprising a plurality of layers may have a thickness of up to 60 μm and more.

[0015] Such a coating, also known as a multi-layer coating, could, for example, be built up as follows: one or more layers based on titanium, a layer of aluminium oxide in the α and/or κ modification and optionally one or more covering layers based on titanium, wherein at least between two layers based on titanium an intermediate layer is arranged.

[0016] Each of the layers, except the intermediate layer, may, depending on the material used, have a thickness of approximately 10 μm, for example in the case of a layer based on titanium, up to approximately 20 μm, for example in the case of a layer of aluminium oxide, such that layer thicknesses of over 60 μm may be obtained. In order to guarantee the adhesion strength of a coating, for example of such a thickness, at least one intermediate layer is necessary between the layers based on titanium.

[0017] The invention advantageously affords the possibility of providing every substrate material, the base material of the tool, whether it be high-speed steel, hard metal or also ceramic materials such as, for example, silicon nitride, SiAlON, aluminium nitride or based on Al₂O₃, with a coating which adheres well and substantially reduces wear. Here, the substrate material may be selected in accordance with the desired characteristics.

[0018] The invention is explained in greater detail by reference to an Embodiment Example.

[0019] The Embodiment Example shows a schematic representation of a microtome section through a coated structural component 1 made of silicon nitride, Si₃N₄. On the base material 2 of the structural component 1, the sub q t having the light interstices comprising the sintering aids necessitated by production, a coating 3 is located which comprises a plurality of layers, a multi-layer coating. The coating 3 is built up in the present Embodiment Example from four individual layers of differing thicknesses.

[0020] On the substrate 2 a layer 4 follows which is based on titanium, in the present Embodiment Example comprising Ti(C,N). During the coating process, as the layers based on titanium are built up, the following known chemical reactions take place: TiCl₄+½ N₂═TiN+4HCl, TiCl₄+CH₄═TiC+4HCl and TiCl₄+CH₄+½ N₂═Ti (C,N)+4HCl. This is followed, in clearly visible manner, by the intermediate layer 5, predominantly based on cobalt. Whereas the first layer 4 was deposited at a temperature of approximately 1000° C. and at a pressure of 800 mbar, in the case of this layer 5 the pressure was reduced to below 500 mbar at the same temperature. Whereas a cohesively dense part layer 6 of the material component cobalt was deposited first, when the cobalt content is impoverished from the gas to build up the layers based on titanium a part layer 7 forms which comprises predominantly TiC, with interstices 8 of the material component cobalt being still visible. When the coating parameters are changed to the previous state a layer 9 based on titanium is again generated. This layer 9 can be discerned as the lighter colouring. It has the same composition as the first layer, layer 4: Ti(C,N). The last layer 10 is a covering layer of aluminium oxide, Al₂O₃. The coating procedure takes place in accordance with the following known reaction:

2AlCl₃+3H₂O═Al₂O₃+6HCl. 

1. Structural component made of a metallic or ceramic material, in particular for the metal-removing or metal-forming processing of work pieces, which has a coating of hard material, wherein the coating is built up from a plurality of layers and the layers may also comprise different materials, characterised in that at least between two layers (4, 9) of the coating (3) an intermediate layer (5) is arranged which comprises predominantly or exclusively a material component (6, 8) of an element in the fourth to eighth sub-group of the fourth and fifth period of the Periodic Table.
 2. Structural component according to claim 1, characterised in that the intermediate layer (5) is from approximately 0.05 μm to approximately 5 μm thick.
 3. Structural component according to claim 1 or 2, characterised in that the element is an element (6, 8) in the eighth sub-group of the fourth period of the Periodic Table.
 4. Structural component according to one of claims 1 to 3, characterised in that the layers (4, 9) of the coating (3) comprise a material based on titanium.
 5. Structural component according to claim 4, characterised in that the thickness of the layer (4, 9) is up to approximately 10 μm.
 6. Structural component according to one of claims 1 to 3, characterised in that the layer (10) of the coating (3) comprise at least one of the modifications of aluminium oxide.
 7. Structural component according to claim 6, characterised in that the thickness of the layer (10) is up to approximately 20 μm.
 8. Structural component according to one of claims 1 to 7, characterised in that a coating (3) comprising a plurality of layers (4, 9, 10) is built up from an alternating sequence of layers based on titanium (4, 9), and from aluminium oxide (10) in at least one of the modifications thereof.
 9. Structural component according to claim 8, characterised in that a coating of the structural component is built up from a plurality of layers as follows: one or more layers based on titanium, a layer of aluminium oxide in the α and/or κ modification and optionally one or more covering layers based on titanium, wherein at least between two layers based on titanium an intermediate layer is arranged.
 10. Structural component according to one of claims 1 to 9, characterised in that the substrate material (2) of the structural component (1) is silicon nitride or SiAlON.
 11. Structural component according to one of claims 1 to 9, characterised in that the substrate material of the structural component is aluminium nitride.
 12. Structural component according to one of claims 1 to 9, characterised in that the substrate material of the structural component is a material based on Al₂O₃.
 13. Process for the application of a coating of hard material to a structural component made of a metallic or ceramic material as the substrate, in particular for the metal-removing or metal-forming processing of work pieces, wherein the coating may be built up from a plurality of layers, and the layers may also comprise different materials, in particular according to one of claims 1 to 12, characterised in that during application of the layers, preferably by the PVD, CVD or plasma-CVD processes, at least in one layer an element in the fourth to eighth sub-group of the fourth and fifth period of the Periodic Table is likewise deposited from the gas phase, and that the deposition is controlled by the process parameters so that with the material component of the deposited element an intermediate layer is formed which comprises predominantly or exclusively this material component.
 14. Process according to claim 13, characterised in that the quantity of the deposition of the element is controlled by way of varying the pressure.
 15. Process according to claim 14, characterised in that the intermediate layer is formed by reducing the pressure.
 16. Process according to one of claims 13 to 15, characterised in that the thickness of the intermediate layer is controlled by way of the quantity of the element held in readiness for the deposition.
 17. Process according to one of claims 13 to 16, characterised in that the intermediate layer is built up to a thickness of from approximately 0.05 μm to approximately 5 μm.
 18. Process according to one of claims 13 to 17, characterised in that in the case of a coating comprising a plurality of layers on the structural component, in alternating sequence layers based on titanium and layers of aluminium oxide in at least one of the modifications thereof are deposited.
 19. Process according to claim 18, characterised in that in the case of a coating of the structural component comprising a plurality of layers first one or more layers based on titanium, followed by a layer of aluminium oxide in the α and/or κ modification and optionally followed by one or more covering layers based on titanium are applied, and that at least between two layers based on titanium an intermediate layer is deposited.
 20. Process according to one of claims 12 to 17, characterised in that the layers of the coating are deposited, depending on the material, in a thickness of up to approximately 10 μm or up to approximately 20 μm.
 21. Process according to one of claims 13 to 20, characterised in that the coating is preferably deposited on structural components made of one of the materials silicon nitride, SiAlON, aluminium nitride or based on Al₂O₃. 